Silicon carbide (SiC) is a thermally and chemically stable compound semiconductor. Compared with silicon (Si), SiC has an excellent band gap, break down voltage, electron saturation velocity, and thermal conductivity. Therefore, SiC is promising for application in technical fields such as power device materials with low operating loss, high-voltage resistant high-frequency device materials, environment-resistant devices used in high-temperature environments, and radiation-resistant devices. In these technical fields, a high quality SiC single crystal with few crystal defects is demanded.
Methods for manufacturing a SiC single crystal include a sublimation method and a solution growth method. A method for manufacturing a SiC single crystal by the sublimation method is disclosed in, for example, JP2001-72490A (Patent Document 1). Moreover, a method for manufacturing a SiC single crystal by the solution growth method is disclosed in, for example, JP2007-126335A (Patent Document 2).
Compared to the sublimation method, the solution growth method produces a single crystal with less crystal defects. In the solution growth method, a SiC single crystal which is attached to a lower end of a bar-shaped seed shaft is brought into contact with a SiC solution housed in a crucible. Next, the SiC seed crystal is pulled up while the seed shaft is rotated so that a SiC single crystal is grown on the SiC seed crystal. The SiC solution refers to a solution in which carbon (C) is dissolved in a solution of Si or Si alloy.
In the solution growth method, the temperature of the portion of the SiC solution in the periphery of the SiC seed crystal (hereafter, referred to as a SiC seed-crystal peripheral area) is kept lower than that of the other portion of SiC solution. This will cause a supersaturation of SiC in the SiC seed-crystal peripheral area, thereby accelerating the growth of the SiC single crystal.
However, when the SiC seed-crystal peripheral area is excessively cooled, a SiC polycrystal, instead of a SiC single crystal, is likely to be generated in the vicinity of the SiC seed crystal. The generated SiC polycrystal is moved to the SiC seed crystal by the flow of the solution. When a large amount of SiC polycrystal adheres to the SiC single crystal which has grown on the SiC seed crystal, the growth of the SiC single crystal may be hindered.
JP2004-323247A (Patent Document 3), JP2008-100854A (Patent Document 4), and JP2006-131433A (Patent Document 5) disclose methods for manufacturing a SiC single crystal for the purpose of suppressing the generation of SiC polycrystal.
In the manufacturing method disclosed in Patent Document 3, an insulating cover or a graphite cover is disposed above the solution surface to suppress the heat release from surface of the SiC solution.
In the manufacturing method disclosed in Patent Document 4, the SiC seed crystal attached to the lower end of a pull-up shaft is held at a position deviated from the axial centerline of the pull-up shaft. Then, at the time of manufacturing a single crystal, the pull-up shaft is rotated around the axial centerline. In this case, the SiC seed crystal circles around in the solution. The document describes that since this will cause the SiC seed crystal to be constantly in contact with an appropriately supersaturated solution, the generation of SiC polycrystal is suppressed.
In the manufacturing method disclosed in Patent Document 5, an insulating structure is disposed in a free space above the crucible to adjust an in-plane temperature difference in the free surface of the solution to be within 40° C.